As a radio wave lens for an antenna device, there is known one that uses a Luneberg lens. The Luneberg lens is a spherical lens made of dielectric material, wherein the relative dielectric constant varies within a range from 2 to 1 or its approximate value from the center of the sphere to the outer periphery. Further, there is another type of Luneberg lens that achieves the function equivalent to that of the spherical lens by combining a hemispherical lens with a radio wave reflector having a greater size than the hemispherical lens (see, e.g., Patent Document 1).
Since the radio wave lens antenna disclosed in Patent Document 1 uses a hemispherical lens, and therefore, the size can be reduced and the cost can be saved compared to the case of using a spherical lens. However, since it is configured such that its entire parts are covered with a radome for protection, the size becomes large. Further, and the radome of a hollow structure must have a large thickness to secure a sufficient strength, which causes problems in electric characteristics and an increase in cost.
Regarding this, in the radio wave lens antenna of the structure disclosed in Patent Document 1, a lens cover of a hemispherical shell shape may be used such that the lens is sealed by the lens cover and a reflector. Since the lens cover is in contact with the surface of the lens, the size and the thickness can be made smaller. Thus, a further reduction in size can be achieved, and desirable electric characteristics can be acquired more easily compared to the antenna that uses a radome.
However, Patent Document 1 does not mention anything about the fixing and liquid sealing of the lens. The lens is usually fixed to the reflector by using an adhesive. However, the adhesive may be deteriorated after a long period of use, and thus the lens may be detached therefrom. Also, the lens may be removed due to an impact, wind pressure, bending of the reflector by vibration, or the like. In this case, a gap in which the dielectric constant differs from that of the lens may be formed between the lens and the reflector, thereby greatly degrading the electrical performance of the antenna device. Furthermore, when the adhered portion is peeled off while the lens cover is misaligned or damaged, there is a risk of the lens falling down.
Further, if the reflector is not properly sealed to the lens cover, rainwater, moisture or the like may penetrate the inside of the lens cover. Since water has a high dielectric constant (∈r) and a high dielectric loss (tan δ), merely a slight amount of moisture that has seeped into the lens may sharply degrade the electrical performance of the antenna device. However, Patent Document 1 does not disclose any solution to these problems.
Patent Document 1: Japanese Patent Application Publication No. 2002-232230